This invention relates to fluid handling. There are applications where it is advantageous to engage or disengage piston rods to control the flow of a fluid. There are also applications where more than one fluid is needed to be metered and delivered at a time. A few such applications are in sample preparation instruments for blood, serum and urine. Reagents and other chemicals are added to the sample to prepare it for analysis. Heretofore, there were two common systems to add more than one fluid to the sample. The first means was to have a separate motor and pump for each fluid to be added. This provided a lot of flexibility but at a high price. It also made the apparatus complex, large and heavy. The second means was to use one motor and pump but to equally stroke a mechanically connected bank of syringes. The diameter of the syringes in the bank could be varied so they could deliver different flow rates or volumes for the same stroke. This costs less than the first means but is much less flexible and makes it difficult and time consuming to vary which fluids are being delivered and in what volumes because syringes must be manually changed or removed. Accordingly it would be desirable to select which syringes will deliver fluid, to easily and quickly change the syringes and to have a smaller, less complex and lower cost drive means.